1. Field of the Invention
The present invention relates to a high frequency oscillator for oscillating signals in a millimeter-wave band and a microwave band, and more particularly, to a high frequency oscillator which employs a transmission line resonator and is suitable for generation of a frequency four times or more higher than a fundamental oscillation frequency (i.e., fundamental wave) based on the resonator.
2. Description of the Related Art
High frequency oscillators suitable for generating oscillation signals in a millimeter-wave band and a microwave band are employed, for example, in optical communication systems and peripheral apparatuses associated therewith, and are required to improve the performance and reduce the cost. The present inventors have previously proposed such a high frequency oscillator which is a push-push oscillator using a transmission line resonator in Japanese Patent Laid-open Publication No. 2003-152455 (JP, P2003-152455A). This oscillator can generate signals at frequency twice or four times higher than a fundamental resonant frequency of the resonator in a simple structure. It should be noted that the push-push oscillator employs a pair of oscillation circuits which operate at the same fundamental frequency and in opposite phases to each other, and combines outputs of these oscillation circuits to cancel out fundamental wave components and extract even-order harmonic components to the outside.
FIG. 1 is a plan view illustrating the configuration of a conventional second-harmonic oscillator for generating a second harmonic signal which is a signal at a frequency twice as high as a fundamental resonant frequency of the oscillator.
This high frequency oscillator comprises transmission line resonator 1, active devices 2 for oscillation, and an output line 3. Transmission line resonator 1 is microstrip line resonator 1A comprised, for example, of a microstrip line, and has an elongated signal line routed on one principal surface of dielectric substrate 4. A ground conductor is disposed substantially over the entirety of the other principal surface of dielectric substrate 4. Microstrip line resonator 1A has a length of, for example, λ/2, where λ is the wavelength corresponding to a fundamental frequency (fundamental wave f0) of oscillation.
Each of active devices 2 for oscillation is comprised, for example, of FET (Field Effect Transistor) 2A. In the illustrated oscillator, FETs 2A are disposed at both ends of microstrip line resonator 1A, respectively. Each FET 2A has a gate connected to a corresponding end of microstrip line resonator 1A through capacitor 5 for providing loose coupling. By thus connecting the gates of FETs 2A, microstrip line resonator 1A has resonant wave points (i.e., antinodes or nodes of a standing wave) at both ends.
The gate of each FET 2A is applied with a bias voltage through another microstrip line, not shown. Each FET 2A has a drain connected to microstrip line 6a which is designed to provide a negative resistance, and supplied with an operating voltage from a power supply, not shown. Each FET 2A has a source grounded. Microstrip line 6a is configured to have an electrically open end, when viewed from the drain, with respect to the fundamental oscillation frequency.
Output line 3, which has a microstrip line structure, is connected to a midpoint of microstrip line resonator 1A in the longitudinal direction through capacitor 5 for providing loose coupling. It should be noted that by loosely coupling output line 3 and FETs 2A to microstrip line resonator 1A, microstrip line resonator 1A is improved in independence, and is prevented from being affected by FETs 2A and output line 3. Connected at a midpoint of microstrip line 1 is microstrip line 6b which has a length of λ/4, and an open distal end. This microstrip line 6b, which is a so-called quarter wavelength stub, functions as an electrically short-circuited point with respect to a fundamental wave component f0, when viewed from a point at which it is connected to the resonator.
In this high frequency oscillator, equivalently, active devices 2, each comprised of FET 2A and having a negative resistance, are connected to both ends of microstrip line resonator 1A through electric field coupling. Therefore, for example, as illustrated in FIG. 2, a standing wave of fundamental wave f0 is generated with a null potential point located at the midpoint of microstrip line resonator 1A, and maximum displacement distribution points located at both ends of microstrip line resonator 1A which are resonant wave points, at which the standing wave is in an opposite phase relationship to each other. Also, other standing waves are generated corresponding to n-th harmonic n×f0 of fundamental wave f0, where n is an integer equal to or larger than two. In FIG. 2, the standing wave of fundamental wave f0 is represented by a solid line; a standing wave of second harmonic wave 2f0 is represented by a broken line; and a standing wave of third harmonic wave 3f0 is represented by a one-dot chain line. Among these standing waves, fundamental wave f0 has the highest level, and the level becomes lower as harmonics have higher order numbers.
In the foregoing configuration, since the negative resistive devices (FETs 2A) are disposed at both ends of microstrip line resonator 1A, i.e., resonant wave points, there are formed two oscillation systems which share the microstrip line resonator for fundamental wave f0 and higher-harmonics n×f0 components. Since both ends of microstrip line resonator 1A are reverse-level potential points to each other in the oscillation system associated with fundamental wave f0, the two oscillation systems oscillate in opposite phases to each other. Then, since output line 3 is connected to the midpoint of microstrip line resonator 1A, which is an electrically short-circuited point, i.e., a null potential point with respect to fundamental wave f0, fundamental wave f0 is not delivered from output line 3. Also, for third harmonic or higher harmonic components (2n−1)×f0, i.e., odd-order harmonics of fundamental wave f0, the midpoint of microstrip line resonator 1A serves as a null potential point, thus preventing these harmonic components from being delivered from output line 3, in a manner similar to fundamental wave f0.
The standing wave of second harmonic 2f0 relative to fundamental wave f0, has maximum displacement distribution points at the midpoint and both ends of microstrip line resonator 1A at which the standing wave is in opposite phases to each other. Since output line 3 is connected to such a midpoint, second harmonic 2f0 is generated at output terminal fout which is the other end of output line 3. In this way, the high frequency oscillator can provide an oscillation frequency which is twice higher than fundamental wave f0.
A standing wave of fourth or more even-order harmonic 2n×f0 also has a maximum displacement distribution point at least at the midpoint of microstrip line resonator 1A, and is in opposite phase or in phase to the midpoint at both ends. As a result, a fourth or more even-order harmonic is similarly generated from output line 3, but has an output level relatively low as compared with the second harmonic component.
FIG. 3 illustrates the second-harmonic oscillators, illustrated in FIG. 1, arranged in parallel on the same substrate 4, and a microstrip line (first output line 3A) having a length of λ/4, where λ is the wavelength of fundamental wave f0, disposed between the second-harmonic oscillators. First output line 3A has one end connected to a midpoint of microstrip line resonator 1A through capacitor 5 for loose coupling, and the other end connected to a midpoint of the other microstrip line resonator 1A through capacitor 5 for loose coupling. In other words, first output line 3A has a length equivalent to one-half wavelength with respect to a second harmonic component 2f0, and connects the midpoints of both microstrip line resonators 1A to each other. Further, second output line 3B is connected to a midpoint of first output line 3A through capacitor 5 for loose coupling. Further connected at the midpoint of each microstrip line 1A is microstrip line 6b which has an open distal end, but unlike the one illustrated in FIG. 1, microstrip line 6b has a length of λ/8, which is one quarter of the wavelength of second harmonic 2f0 wave.
In the foregoing configuration, second harmonic 2f0 is delivered to first output line 3A from each second-harmonic oscillator, while fundamental wave f0 is suppressed. Then, first output line 3A functions as a half-wavelength resonator for second harmonic 2f0, and similar to the foregoing case, the midpoint of first output line 3A serves as a null potential point with respect to the standing wave of second harmonic 2f0. On the other hand, the midpoint of first output line 3A serves as a maximum displacement distribution point for the standing wave of fourth harmonic 4f0. For this reason, a second harmonic 2f0 component is also suppressed, so that fourth harmonic 4f0 and higher even-order components are only generated on second output line 3B. In this event, since fourth harmonic component 4f0 is prominent among other even-order harmonic components, the circuit illustrated in FIG. 3 functions as a fourth-harmonic oscillator for generating the fourth harmonic, which has a frequency four times as high as that of fundamental wave f0.
As described above, it is possible to implement a high frequency oscillator for generating 2n-th harmonic component 2n×f0, where n is an integer equal to or larger than two, by repeatedly combining two high frequency oscillators for generating a lower order harmonic to form a higher-order harmonic oscillator.
While microstrip line resonator 1A has been described to be linear in the foregoing discussion, the oscillator also operates in a similar manner when microstrip line resonator 1A is formed into an annular or ring shape.
The high frequency oscillators configured as described above, however, have the following problems.
Basically, a second-harmonic oscillator can be made up of one microstrip line resonator 1A having a length of λ/2, and two negative resistive devices at both ends thereof, where λ is the wavelength corresponding to the fundamental frequency of oscillation, i.e., fundamental wave f0. However, since a fourth-harmonic oscillator requires a pair of second-harmonic oscillators which cause the resulting oscillator to be large in size, and therefore hampers a reduction in size of the oscillator. Likewise, for providing a 2n-th harmonic oscillator for generating an eighth harmonic or a higher-order harmonic (n≧3), a pair of 2(n−1)-th harmonic oscillators are required, and further hamper a reduction in size of the oscillator.